
The core-degenerate (CD) scenario involves the merger of a CO, or HeCO, WD with the core of a massive asymptotic giant branch (AGB) star. The merger remnant is a WD close to the Chandrasekhar mass limit that explodes at a later time. The time to explosion is the merger to explosion (MED) delay time \(t_{\rm MED}\)  (e.g., [1]}, [2]}, [3]}). The possibility that the CO WD merger remnant maintains a very small mass of helium and the possible consequences are the subject of the present study.

The double degenerate (DD) scenarios and the DD-MED scenario involve the merger of two WDs (e.g., [4]}, [5]}) that lose energy to gravitational waves. Helium might play a crucial role in igniting the merging WDs (e.g., [6]}, [7]}, [8]}). The explosion might take place during the merger itself (e.g., [9]}, [10]}, [11]}), or at a MED time (e.g., [12]}, [13]}, [14]}, [15]}, [16]}, [17]}).

In the double-detonation (DDet) scenario ignition of helium that a CO-rich WD accretes from a companion triggers the WD explosion (e.g., [18]}, [19]}, [20]}).

In the single degenerate (SD) scenario a CO WD accretes hydrogen-rich gas from a non-degenerate companion (e.g., [21]}, [22]}, [23]}, [24]}, [25]}). The WD might explode as soon as it approaches the Chandrasekhar-mass, or it might explode after a long delay (long MED) after losing angular momentum (e.g., [26]}, [27]}, [28]}).

In the WD-WD collision (WWC) scenario the two WDs explode as they collide at about their free fall velocity (e.g., [29]}, [30]}, [31]}, [32]}). Studies (e.g., [33]}, [34]}, [35]}) found that the WWC scenario contributes \(<1 \%\)  of all SNe Ia.

